GRD product from S1: wrong georeference

[alankingnz]

Hi all, I’m new to this community. I have a problem with the SAR scene from S1: it rotated 90 degrees, not the North oriented like the map

It should be like this

I do not know if this rotation will affect other processing step or not. Please help me !

Kind regards,
Alan

[kutao]

Hi Alan,

did you already perform a terrain correction on the image?
If not, please be aware of the specifics of SAR acquisition geometry; Sentinel-1 is a so-called right-looking satellite acquires images in ascending and descending orbit. Depending on the current orbit from the scene, the image appears “flipped”.

You can find more information in this related post:

How can Sentinel-1 images be un-flipped?

Where is the looking side (left or right) stored in Sentinel-1 GRD images metadata? From the Sentinel Online FAQ: Some Sentinel-1 images may appear mirrored when opened in the Sentinel-1 toolbox. This is an effect of the acquisition geometry: the mirroring effect may be present or not depending on the combination of orbit (descending or ascending) and looking side (left or right). If an image is mirrored it is possible to flip it with the NEST toolbox using the Data Flip tool in the Utilities…

For more information on the technical properties, please have a look at this detailed description of the technical properties of Sentinel-1:

Copernicus: Sentinel-1 - eoPortal

For more “starter information” you can also have a look here: https://saredu.dlr.de/
And here, an online course is starting next month on the topic of an “Introduction to Radar Remote Sensing”: https://eo-college.org/courses/echoes-in-space/
It may seem a little much when you’re just starting in the radar world, but just keep looking up things you don’t understand. We all started from scratch

Welcome to the community!

Regards,
Denise

[mengdahl]

Please go through the tutorials.

[alankingnz]

Thanks Denise,

Before seeing your answer, I applied the “Geolocation Grid” correction and the img seemed to flip back to how it should be. However, after that operation, I could not apply Radiometric Flattening, error says that “Source product should not be map projected”.

I have not found anywhere to use the Flip. I think it is in NEST toolbox rather than SNAP/S1Tbx. I will try the Range Doppler Terrain Correction to see if the img be flipped back.

I have put my name in the EO-College course already.

What I intend to do with S1 SAR: The cloudy weather blocks me from having good Sen2 img. I used those images to visually detect the forest cutover on the ground. Now I hope with S1 SAR I can somehow see through the “cloud” to see those cutovers.

Our cutover site sometimes can be small 100m x 100m but I still can see it on Sentinel 2. I’m not sure if that dimension is too small for Sentinel 1 ?

Thank you very much for the support !

Regards,
Alan

[mengdahl]

Please go through the tutorials for S1TBX. Use Range-Doppler terrain correction to orthorectify the image into map coordinates.

[nuno.miranda]

Dear Alan,

For what concern your primary request on the fact the images are not properly oriented.
I have given an answer to that on that post:

How can Sentinel-1 images be un-flipped?

Dear carlodef, You should recall that S-1 is a radar. The geometry on which the data is provided is the most simple one if you think at it from a radar point of view. It is a collection of echoes ordered in time and this both in azimuth and range. This is the natural geometry of the radar and the most simple one to work with to fully exploit the product. The side effect is that indeed depending on the pass (ascending/descending) your image might appear north/south (ascending) flipped or east/…

Please have a look to it.

kind regards,
Nuno Miranda